The present invention relates to a TV camera tube of the so-called HN type, that is, a high-electron-beam-velocity scanning and negative charging type, particularly, a photoconductive type TV camera tube having a metal mesh and a collector electrode arranged on the electron beam scanning side of a negative-charged photoconductive target and camera circuitry for deriving a camera output signal therefrom.
A TV camera tube of the HN type is disclosed by Miyashiro in "TV camera tube having a negative-charged target scanned by a high speed beam", Television, Vol. 19, No. 2, 1965, pp 96-102.
Comparing this TV camera tube of the HN type with the conventional LP type, namely, the low-electron-beam-velocity scanning and positive charging type, which has a positive-charged photoconductive target scanned by a low speed electron beam, the latter is operated at a low target voltage and a secondary electron emission ratio .delta. which is smaller than unity, namely, .delta.&lt;1, so that scanning electrons land directly on the target. In other words, the scanning beam gives a negative charge to the target. As a result, a current of electrons is circulated in a direction from a cathode to the cathode through the target and a signal electrode successively.
On the other hand, in the TV camera tube of the HN type, which is quite different from that of the LP type, the target is scanned at a high target voltage so that .delta.&gt;1. As a result, secondary electrons emitted from the target by the scanning beam are collected by a collector mesh electrode having a voltage applied thereto which is a few volts higher than that of the target. In this situation where .delta.&gt;1, the secondary electrons, which are more than the electrons of the scanning beam running into the target, are collected by the collector mesh electrode, so that the scanning beam gives a positive charge to the target. In other words, the current of electrons flows in a direction from the target to the signal electrode through the collector mesh electrode. Accordingly, the direction of the current of electrons flowing through the target is opposite to each other between the LP type and the HN type.
In connection therewith, it is confirmed that a TV camera tube of the HN type has the following advantages in comparison with that of the LP type.
(1) The capacitive discharge lag performance is better.
(2) The resolution performance is better.
(3) The energy of the scanning beam is higher, so that beam bending is scarcely caused.
Therefore, there is demand for the development of a target which is suitable for a camera tube of the HN type, in other words, which is hardly damaged by the high speed electron beam having high energy and is operated at a polarity opposite that scanned by the low speed electron beam. However, such a target has not yet been realized and how to manufacture it has not yet been determined, and further a suitable method for deriving a picture signal therefrom has not yet been investigated.
The above situation is based on such problems as the following: